Process for making tetraalkyl lead compounds



United States Patent 3,116,308 PROCESS FOR MAKING TETRAALKYL LEAD COMPOUNDS Jack Linsk, Highland, Ind., assignor to Standard Oil Company, Chicago, Ill., a corporation of Indiana No Drawing. Filed June 13, 1961, Ser. No. 116,685 7 Claims. (Cl. 260-437) This invention relates to organometallic compounds, and more particularly concerns an improved electrolytic process for making tetraalkyl lead compounds.

It has recently been discovered and disclosed that tetraalkyl lead compounds such as tetraethyl lead may be prepared by electrolyzing the corresponding alkyl Grignard reagent with a lead anode. By this procedure, alkyl groups from the Grignard reagent are transferred to the anode, where they form tetraalkyl lead.

There are however certain problems associated with the electrolytic process. One of these concerns the practical difficulty of conducting the electrolysis to completion. It has been found that an alectrolysis which is con ducted much beyond about 90 percent Grignard reagent conversion results in an electrolyte exhibiting high resistance and which is quite viscous and difiicult to handle. Furthermore, it appears that excessively high conversions lead to the formation of undesirable by-products, namely olefin gases and finely divided metallic lead.

It has heretofore been the practice to avoid the foregoing difficulties by terminating electrolysis after about 9095 percent Grignard reagent conversion and thereupon commence the recovery of tetraalkyl lead product. In general, most of the recovery schemes which have been proposed have entailed hydrolysis of unconverted alkyl Grignard reagent with Water or with a dilute aqueous acid. This procedure has the feature of avoiding most of the aforementioned cell difficulties, and in addition can enable tetraalkyl lead compounds to be distilled from the electrolytic reaction mixture without exposing reactive Grignard reagents to high temperatures. However, as a concomitant disadvantage, the hydrolyzed Grignard is thus Wasted.

It is therefore a principal aim of the present invention to provide an improved method of recovering tetraalkyl lead compounds from the aforementioned electrolytic reaction mixtures in a manner which retains the process advantages of Grignard reagent hydrolysis, and which at the same time enables unconverted Grignard reagent to be converted to additional tetraalkyl lead compound.

Briefly, and in accordance with the invention, the inventive method is applied to processes for the preparation of a tetraalkyl lead compound wherein an alkyl Grignard reagent is electrolyzed with a lead anode and wherein electrolysis is terminated when all but a minor portion of the alkyl Grignard reagent is converted to the corresponding tetraalkyl lead compound. Then, instead of hydrolyzing the unconverted alkyl Grignard reagent, the reagent is reacted with a suitably reactive lead compound to form additional tetraalkyl lead compound by chemical action. Thereafter, both electrolytically-produced and chemically-produced tetraalkyl lead compounds may be recovered in conventional manner.

By the foregoing procedure it is evident that all of the advantages of Grignard reagent hydrolysis, viz. avoidance of cell difiiculties and prevention of exposure of alkyl Grignard reagents to high temperatures, are obviated, while at the same time unreacted alkyl Grignard reagent is converted to useful product.

The electrolytic preparation of tetraalkyl lead compounds such as tetraethyl or tetramethyl lead has been Well described in patent literature. Briefly, an alkyl Grignard reagent, i.e. an alkyl magnesium chloride, bromide, or iodide, while in a suitable ether or tertiary 3,116,308 Patented Dec. 31, 1963 'ice amine, is electrolyzed using a lead anode. Advantageously a minor amount of free alkyl halide is also present in the electrolyte. By electrolysis, alkyl groups in the Grignard reagent are transferred to the anode, so that the over-all reaction is that two mols of alkyl Grignard reagent, two mols of alkyl halide, and one mol of metallic lead form one mol of tetraalkyl lead and two mols of by-product magnesium dihalide. Various ethers and/or tertiary amines may be employed, and these include, by way of illustration, diethyl ether, dimethyl ether, dialkyl ethers of the ethylene or polyethylene glycols, having at least two carbon atoms in each alkyl group and not more than two ethylene groups in the glycol portion such as the diethyl ether of ethylene glycol or of diethylene glycol, the dibutyl ether of diethylene glycol, and the Ihexylethyl ether of diethylene glycol; triethyl amine, tributyl amine, etc. Various ethers or non-ether materials may be added to the electrolyte to improve electrical conductivity, and these include normally liquid aromatic hydrocarbons such as benzene, toluene, or xylenes, or tetrahydrofuran.

Alkyl Grignard reagent concentration, prior to the onset of electrolysis, may vary over wide limits, advantageously being within the range of about 1.5 to about 3.5 normal. When, as is preferred, an excess of alkyl halide is present, this concentration may range from about 1 to about 10 weight percent, preferably from about 1 to about 3 weight percent, of the electrolyte.

Conditions in a typical electrolytic cell depend largely on the nature of the electrolyte and its individual components. Electrolyses have been conducted at temperatures as low as 20 C. and even lower and as high as C. and even higher, but are optimally within the range of about 20 to about 50 C. Current densities, at both anode and cathode, are advantageously within the range of about 0.2 to about 100 amperes per square foot, and relatively low voltages, of the order of about 20-30 volts, are preferred, although voltages may range from 3 or 4 volts to 50 volts or even higher. Cell pressures may range from atmospheric to low superatmospheric, say 60 p.s.i.g. but are preferably less than about 30 p.s.i.g.

The electrolysis is advantageously continued in either a batch-wise or continuous process until all but a minor portion of the alkyl Grignard reagent is converted to the corresponding tetraalkyl lead compound. Advantageously and most preferably, all but about 1 to 10 percent of the alkyl Grignard reagent is converted electrolytically before the present chemical conversion of unconverted reagent is employed.

After electrolysis has continued to the desired extent, and when no further electrolysis is deemed justified in light of the difiiculties attendant upon electrolyzing to an overly great extent, the inventive procedure of reacting unconverted alkyl Grignard reagent with a suitably reactive lead compound is commenced. Parenthetically, the electrolyzed reaction mixture may first be treated in one or more process steps, as for example by flash distillation or by low temperature subatmospheric distillation to remove volatile components such as excess alkyl halide and low boiling ethers before the inventive reaction step is employed.

The reaction between alkyl Grignard reagents and reactive lead compounds is per se known, and no claim to novelty of this step, in and of itself, is herein asserted. Thus, for example, an article by Pearson et al., appearing at pages 299-305 of the monograph Metal-Organic Compounds, Advances in Chemistry Series 23, describes the suitability of such reactive lead compounds as plumbous chloride, lead monoxide, lead dioxide, lead acetate, lead fo-rmate, lead sulfide, lead salts of inorganic and organic thioacids and oxyacids such as lead sulfate, lead nitrate, and lead thiocyanate, lead tetraacetate, lead stearate, lead oxalate, lead naphthenate, and lead linoresinate.

Various catalysts for the reaction are likewise known, and include iodides such as an alkyl iodide, e.g. methyl iodide and iodine (U.S. 1,105,723), hydroiodides of an aromatic amine or of an iodine substitution product of an aromatic amine (U.S. 1,863,451), etc. (V-ide US. 1,690,075, US. 1,798,593, and US. 1,805,756).

Conditions for the chemical reaction are similarly Well known. Temperatures have ranged from as little as C. to as high as 150 C. and higher, and advantageously are within the range of about 20 to about 120 C. Mol ratios of alkyl Grignard reagent to reactive lead compounds advantageously range from about 0.3 to 3.0, but may be considerably lower or higher depending upon particular circumstances, i.e. the cost and/or value of the Grignard reagent and/or reactive lead compound in relation to the value of additional recoverable tetraalky l lead compound. Reaction times normally vary from about 0.1 to about hours, all more or less, depending upon the particular Grignard reagent and lead compound, temperature, etc. Also, it has been found of advantage to maintain an excess of the appropriate alkyl halide in the reaction mixture as this apparently increases the yield of tetraalkyl lead compound.

The procedure according to the invention permits a substantial increase in recoverable tetraalkyl lead product. Ordinarily, it can be expected that from 20 to as much as 70 percent, or even more, of the unconverted alkyl Grignard reagent remaining after electrolysis, and which would normally be hydrolyzed and wasted, can be converted to useful tetraalkyl lead compound. It will be evident that this represents a substantial increase in tetraalkyl lead compound production at no expense to the electrolytic portions of a plant.

After chemical reaction of the alkyl Grignard reagent and lead compound, the electrolytic reaction mixture can then be treated by any of the procedures known to the art for recovering tetraalkyl lead compound. By way of illustration, the mixture may be distilled to separate its components according to their boiling point; distillation advantageously is conducted at atmospheric or subatmospheric pressure.

To more fully illustrate the practice of the invention, the following specific example is presented. It will be understood that this is by way of illustration only and is not to be considered wholly exclusive or definitive with respect to scope and/ or conditions.

Illustrative Example The electrolytic cell is provided with a plurality of lead plate anodes and stainless steel cathodes which are connected to a source of direct current; the total anode area and cathode area are each about 1 square foot. Provisions are made for withdrawing electrolyte during electrolysis, passing this through a cooling system, and recirculating electrolyte back to the cell.

A 1.98 molar methyl magnesium chloride solution is prepared; the solution contains 9.8 weight percent tetrahydrofuran, 2 percent excess methyl chloride, 46.2 percent benzene, and the balance (exclusive of Grignard reagent) being hexylethyl ether of diethylene glycol. During electrolysis, additional methyl chloride is charged to the cell in an amount corresponding to 5.3 weight percent of the initial electrolyte.

Electrolysis is commenced batchwise, and is continued for a period of 11 hours at an average temperature of about 41.1 C. The average pressure is 3.1 p.s.i.g., while the average current flow is 20.0 amps.

At the end of 11 hours of electrolysis, a total of 219.6 ampere hours of current has passed through the cell. 95.3 percent of the Grignard is converted to tetramethyl lead in a yield exceeding 99 percent. The current efficiency is 158 percent of theoretical, and the power con- 4 surned is 2.86 kilowatt hours per pound of tetramethyl lead produced.

The electrolyzed reaction mixture is discharged from the cell and placed in a glass reaction vessel which has previously been freed of oxygen. Plumbous chloride, PbCl in a proportion of 1 mol of plumbous chloride per mol of unreacted (computed) alkyl Grignard reagent,

is charged to the reactor vessel, and heating is commenced ether of diethylene glycol.

An excellent tetramethyl lead product recovery is obtained, which is considerably higher than that which would be obtained had unconverted alkyl Grignard reagent been hydrolyzed according to the prior art procedures.

From the foregoing description it is evident that there has been provided an extremely useful and valuable technique for increasing the recovery of tetraalkyl lead compound from processes which employ the electrolysis of the corresponding alkyl Grignard reagent with a lead anode. Many variations, alternatives, and modifications will of course be evident to those skilled in the art in light Otf the foregoing description.

It has been reported in the literature that if the reaction of reactive lead compounds with certain alkyl Grignard reagents is conducted at an inordinately low temperature, instead of tetraalkyl lead there is obtained hexylalkyl dilead; should this latter product be obtained, it is only necessary to increase the temperature at which chemical reaction is efifected.

Thus having described the invention, what is claimed is:

1. In a process for the preparation of a tetraalkyl lead compound wherein an alkyl Grignard reagent is electrolyzed with a lead anode and wherein electrolysis is. terminated when all but a minor portion of said alkyl Grignard reagent is converted to the tetralkyl lead compound, the improvement comprising reacting said unconverted alkyl Grignard reagent with a reactive lead compound selected from the group consisting of plumbous chloride, lead monoxide, lead dioxide, lead acetate, lead formate, lead sulfide, lead sulfate, lead nitrate, lead thiocyanate, lead tetraacetate, lead stearate, lead oxalate, lead naphthenate, and lead linoresinate to form additional tetraalkyl lead compound, and recovering the total tetraalkyl lead compound thus, produced.

2. Process of claim 1 wherein said reaction is efi'ected when all but one to ten percent of the alkyl Grignard reagent is converted electrolytically.

3. Process of claim 1 wherein said reactive lead compound is plumbous chloride.

4. Process of claim 1 wherein said reaction is effected in the presence of excess alkyl halide.

5. Process of claim 1 wherein said reaction is effected in the presence of an iodide catalyst.

6. Process of claim 1 wherein said tetraalkyl lead compound is tetraethyl lead.

7. Processof claim 1 wherein said tetraalkyl lead compound is tetramethyl lead.

References Cited in the file of this patent Chemical Reviews, vol. 2, 1925, pp. 43 to 54.

Patent No., 3 116 308 December 31, 1963 Jack Linsk It is hereby certified that err ent requiring correction and that th corrected belo' or appears in the "above numbered pate said Letters Patent should read as Column 1 line 21 for "electrolysis" read electrolysis column 3 line 3 for "(U. S, 1,, 105, 723) read (U. S. 1,705 723) Signed and sealed this 26th day of May 1964,

(SEAL) Attest:

ERNEST W SWIDER Attesting Officer EDWARD Jo BRENNER Commissioner of Patents 

1. IN A PROCESS FOR THE PREPARATION OF A TETRAALKYL LEAD COMPOUND WHEREIN AN ALKYL GRIGNARD REAGANT IS ELECTROLYZED WITH A LEAD ANODE AND WHEREIN ELECTROLYSIS IS TERMINATED WHEN ALL BUT A MINOR PORTION OF SAID ALKYL GRIGNARD REAGENTS IS CONVERTED TO THE TETRALKYL LEAD COMPOUND, THE IMPROVEMENT COMPRISING REACTING SAID UNCONVERTED ALKYL GRIGNARD REAGENT WITH A REACTIVE LEAD COMPOUND SELECTED FROM THE GROUP CONSISTING OF PLUMBOUS CHLORIDE, LEAD MONOXIDE, LEAD DIOXIDE, LEAD ACETATE, LEAD FORMATE, LEAD SULFIDE, LEAD SULFATE,LEAD NITRATE, LEAD THIOCYANATE, LEAD TETRAACETATE, LEAD STERATE, LEAD OXALTE, LEAD NAPHTHENATE, AND LEAD LINORESINATE TO FORM ADDITIONAL TETRALKYL LEAD COMPOUND, AND RECOVERING THE TOTAL TETRAALKYL LEAD COMPOUND THUS PRODUCED. 